1. Field of the Invention
The present invention relates to a technique of reading data from a memory.
2. Description of the Background Art
For the supply of a ROM (Read Only Memory) in which program code is written, the specifications may specify not only the program code itself but also the data stored in the entire ROM. One of the purposes thereof is to previously avoid problems that would be caused when data is read from a memory area not intended for access by the user of the ROM.
FIG. 12 is a diagram conceptually showing an example of a specification 100 of a ROM in which program code is written. Such a specification 100 often specifies not only the program code CODE but also no-write region information WRGN and a fixed value FXVL. The no-write region information WRGN indicates a region where the program code CODE should not be stored (hereinafter referred to also as a no-write region). The fixed value FXVL is fixed value data to be written to the no-write region, such as 00H, FFH, etc.
FIG. 13 is a conceptual diagram illustrating an address space 200A. Data is written to the address space 200A faithfully according to the specification 100. Specifically, the no-write region information WRGN is written in an information storage region 201, the program code CODE is written in a code region 202, and a plurality of the fixed values FXVL are written in the no-write region 203.
Conventional techniques related to the present invention are disclosed in Japanese Patent Application Laid-Open Nos. 2006-18453 and 9-231800 (1997).
For the supplier of the ROM using a write-type nonvolatile memory, the writing of the fixed value FXVL is a troublesome job because it lengthens the manufacturing time. At the time of shipment, such a ROM using write-type nonvolatile memory, unlike a common mask ROM, requires a longer time to write the program code as the program code is larger-sized. It is also a troublesome job to check the fixed value FXVL in the no-write region 203 in the test conducted prior to the shipment.
FIG. 14 is a conceptual diagram illustrating an address space 200B. The address space 200B stores the no-write region information WRGN and the program code CODE in the code region 202, but no active write operation is applied to the no-write region 203. As a result, unknown values NC exist as values in the no-write region 203.
On the other hand, the user requires a ROM having an address space faithfully based on the specification 100, and the ROM will often be tested on the user side, e.g. by adopting checksum, on the precondition that the fixed value FXVL is stored in the no-write region 203.